1. Field
Example embodiments relate to a nonvolatile memory device, and for example, to a higher-density nonvolatile memory device with self-presence diode characteristics, which may be simpler to manufacture without a transistor and/or a diode, and/or a nonvolatile memory array including the higher-density nonvolatile memory device.
2. Description of Related Art
Semiconductor memory devices may be manufactured to have a higher degree of integration by forming a larger number of memory cells in each unit area, operate at higher speeds, and/or be driven at lower power levels. Accordingly, a vast amount of research has been conducted in order to develop semiconductor memory devices. Accordingly, various types of memory devices have been introduced.
A semiconductor memory device may include many memory cells connected in a circuit. For example, in Dynamic Random Access Memory (DRAM), which may be an example of a semiconductor memory device, a unit memory cell may be composed of a switch and/or a capacitor. A DRAM may be advantageous because a DRAM may have a higher degree of integration and/or may operate at higher speeds. However, when the supply of power to the DRAM is discontinued, all data stored in DRAM may be lost. A flash memory device may be an example of a nonvolatile memory device in which stored data may be maintained if the power supply is discontinued. However, although flash memory devices may have nonvolatile characteristics, for example unlike a volatile memory device, the degree of integration and/or operating speed of flash memory devices may be less than those of DRAM.
Research has been conducted on nonvolatile memory devices, for example research has been conducted on Magnetic Random Access Memory (MRAM), Ferroelectric Random Access Memory (FRAM), Phase-change Random Access Memory (PRAM), and/or Resistance Random Access Memory (RRAM).
In MRAM, data may be stored by changing a magnetization direction in a tunnel junction. In FRAM, data may be stored using the polarization characteristics of ferroelectrics. Both MRAM and FRAM may have advantages and disadvantages, but as described above, there may be a need for a semiconductor memory device that has a higher degree of integration, operates at higher speeds and/or lower power levels, and/or has better data storage characteristics. PRAM may include a resistor and/or a switch (for example a transistor), and may store data using a change in a resistance value caused by a phase change in a specific material. The PRAM may use a chalcogenide resistor that may be changed into a crystalline phase or an amorphous phase by controlling the temperature at which the resistor may be formed. For example, the PRAM may be a memory device manufactured based on the fact that the resistance value in an amorphous phase may be higher than the resistance value in a crystalline phase.
RRAM may use the characteristics (for example, resistance conversion characteristics) that a resistance value may vary according to a bias voltage of a transition metal oxide. FIG. 1A is a diagram of a related RRAM device that may use a resistance conversion material having a general structure.
Referring to FIG. 1A, an n-type semiconductor layer 12a, a p-type semiconductor 12b, an intermediate electrode 13, an oxide layer 14, and/or an upper electrode 15 may be formed on a lower electrode 11. For example an n-type semiconductor layer 12a, a p-type semiconductor 12b, an intermediate electrode 13, an oxide layer 14, and/or an upper electrode 15 may be sequentially formed on a lower electrode 11. The lower electrode 11, the intermediate electrode 13, and the upper electrode 15 may be formed of a general conductive material. The oxide layer 14 may act as a memory node and may be formed of a transition metal oxide with resistance conversion (for example, variable resistance) properties. For example, the oxide layer 14 may be formed of ZnO, TiO2, Nb2O5, ZrO2, or Ni oxide.
Although the oxide layer 14 may have an electrode at both ends of the oxide layer to be able to operate as a memory node, an additional switching structure may be needed to drive a memory device with an array structure. Accordingly, a general memory device may be constructed to be connected to a transistor structure or a diode structure. A manufacturing process of the transistor structure may be complicated when compared to the diode structure, and the manufacturing of a higher-density memory array with the transistor structure may be limited. The diode structure may mainly be employed in a cross-point type structure which may be a simple memory array structure. The n-type semiconductor layer 12a and the p-type semiconductor layer 12b of FIG. 1A may be diode structures.
A silicon diode and/or an oxide diode may be used as the n-type semiconductor layer 12a and the p-type semiconductor layer 12b, respectively. FIG. 1B is an example graph illustrating the electrical density versus bias voltage electrical characteristics of a related silicon diode and a related oxide diode. Referring to FIG. 1B, the silicon oxide may have a comparatively higher electric density to 107 A/cm2, and the oxide diode may have a comparatively lower electric density to 103/cm2. In terms of a related manufacturing process, the diode structure needs to be formed on the lower electrode 11, and for example, on a metal layer, in the RRAM device illustrated in FIG. 1A. However, the silicon diode may be difficult to deposit on a metal layer even though the silicon diode may be deposited on a metal layer at about 1000° C. through laser annealing, and it may be difficult to put the silicon diode to practical use. The oxide diode may be formed on the metal layer but may have comparatively lower electric density than the silicon diode.
A deposition process and/or an etching process needs to be repeatedly performed to form the diode structure in a cross-point type memory device array. For example, in a structure with multiple stacks as shown in FIG. 1A, it may be difficult to manufacture an array-structured memory device that may operate stably while guaranteeing a yield.